What are the -Xms and -Xmx parameters when starting JVM?

The flag Xmx specifies the maximum memory allocation pool for a Java Virtual Machine (JVM), while Xms specifies the initial memory allocation pool.

This means that your JVM will be started with Xms amount of memory and will be able to use a maximum of Xmx amount of memory. For example, starting a JVM like below will start it with 256 MB of memory and will allow the process to use up to 2048 MB of memory:

java -Xms256m -Xmx2048m

The memory flag can also be specified in different sizes, such as kilobytes, megabytes, and so on.

-Xmx1024k
-Xmx512m
-Xmx8g

The Xms flag has no default value, and Xmx typically has a default value of 256 MB. A common use for these flags is when you encounter a java.lang.OutOfMemoryError.

When using these settings, keep in mind that these settings are for the JVM's heap, and that the JVM can and will use more memory than just the size allocated to the heap. From Oracle's documentation:

Note that the JVM uses more memory than just the heap. For example Java methods, thread stacks and native handles are allocated in memory separate from the heap, as well as JVM internal data structures.

Run the command java -X and you will get a list of all -X options:

C:\Users\Admin>java -X
-Xmixed           mixed mode execution (default)
-Xint             interpreted mode execution only
-Xbootclasspath:<directories and zip/jar files separated by ;>
                      set search path for bootstrap classes and resources
-Xbootclasspath/a:<directories and zip/jar files separated by ;>
                      append to end of bootstrap class path
-Xbootclasspath/p:<directories and zip/jar files separated by ;>
                      prepend in front of bootstrap class path
-Xdiag            show additional diagnostic messages
-Xnoclassgc       disable class garbage collection
-Xincgc           enable incremental garbage collection
-Xloggc:<file>    log GC status to a file with time stamps
-Xbatch           disable background compilation
-Xms<size>        set initial Java heap size.........................
-Xmx<size>        set maximum Java heap size.........................
-Xss<size>        set java thread stack size
-Xprof            output cpu profiling data
-Xfuture          enable strictest checks, anticipating future default
-Xrs              reduce use of OS signals by Java/VM (see documentation)
-Xcheck:jni       perform additional checks for JNI functions
-Xshare:off       do not attempt to use shared class data
-Xshare:auto      use shared class data if possible (default)
-Xshare:on        require using shared class data, otherwise fail.
-XshowSettings    show all settings and continue
-XshowSettings:all         show all settings and continue
-XshowSettings:vm          show all vm related settings and continue
-XshowSettings:properties  show all property settings and continue
-XshowSettings:locale      show all locale related settings and continue

The -X options are non-standard and subject to change without notice.

I hope this will help you understand Xms, Xmx as well as many other things that matters the most. :)

-Xms is the initial heap size for the startup, however, during the working process, the heap size can be less than -Xms due to users' inactivity or GC iterations. This is not a minimum required heap size.

-Xmx is the maximum heap size

You can specify it in your IDE. For example, for Eclipse in Run Configurations → VM arguments. You can enter -Xmx800m -Xms500m as

https://i.stack.imgur.com/lJHpH.png

The question itself has already been addressed above. Just adding part of the default values.

As per http://docs.oracle.com/cd/E13150_01/jrockit_jvm/jrockit/jrdocs/refman/optionX.html

The default value of Xmx will depend on platform and amount of memory available in the system.